netfilter: x_tables: merge ip and ipv6 masquerade modules

No need to have separate modules for this.
before:
 text    data   bss    dec  filename
 2038    1168     0   3206  net/ipv4/netfilter/ipt_MASQUERADE.ko
 1526    1024     0   2550  net/ipv6/netfilter/ip6t_MASQUERADE.ko
after:
 text    data   bss    dec  filename
 2521    1296     0   3817  net/netfilter/xt_MASQUERADE.ko

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>